The invention relates to a membrane-electrode assembly, a polymer membrane for a membrane-electrode assembly and a polymer electrolyte fuel cell in the working temperature range up to 250° C. as well as methods for production of same.
For stationary and mobile generation of electricity, e.g., for road vehicles, in space travel, in power plants, etc., fuel cells are being investigated and developed to an increasing extent as a possible source of electric power. A fuel cell is a galvanic element in which chemical energy is converted to electricity directly, i.e., not by way of thermal energy. The individual cell consists of two invariant electrodes between which there is an invariant electrolyte. The fuel cell supplies electric current continuously; the substance that is to be oxidized, i.e., the so-called fuel, such as hydrogen, which is obtained by cleavage of natural gas, methanol, hydrazine, ammonia, etc., and the oxidizing agent, e.g., oxygen, are supplied continuously, and the oxidation products such as water are removed continuously.
At the start of the use of polymer membranes as the invariant solid electrolyte, attention was devoted to proton conducting polymer membranes based on ionomers containing perfluorinated sulfonic acid units, e.g., perfluoroalkyl sulfonic acid polymer electrolytes. Such a polymer membrane is available by the brand name Nafion® from DuPont, for example. Such polymer membranes are heterogeneous systems. The hydrophilic and hydrophobic polymer building blocks form a cluster structure into which water is incorporated as the prerequisite for a high conductivity. In these heterogeneous systems, the charge transport is bound to the liquid phase, i.e., the aqueous phase. The fact that water molecules in these proton conductors function as proton carriers means that the water concentration in the cell must be kept constant. The latter condition is problematic because water is always created as an oxidation product in the reaction and therefore must be removed in a controlled manner. Likewise, the fuel cells operate at temperatures below 100° C. due to the role of the water molecule as a proton carrier, because water is expelled from the polymer membranes at temperatures above 100° C. This would lead to a reduction in conductivity, which is why the polymer membranes are limited to use in a temperature range below 100° C. (O. Savadogo et al., Journal of New Materials for Electrochemical Systems, vol. 1 (1988), pp. 47-66). Another problem with polymer membrane fuel cells based on Nafion is that the catalysts used are particularly sensitive to catalyst toxins such as carbon monoxide in the temperature range below 100° C. and operate less effectively in general.
U.S. Pat. No. 5,525,436 describes a solid polymer electrolyte membrane which includes a proton conducting polymer that has thermal stability up to temperatures of 400° C. The proton conducting polymer may be a basic polymer, preferably polybenzimidazole (PBI) which is doped with a strong acid, preferably sulfuric acid or phosphoric acid. When doped with a strong acid such as sulfuric acid or phosphoric acid, a basic polymer such as PBI leads to a polymer electrolyte which forms a single-phase system in which the acid is complexed by the polymer. In contrast with the heterogeneous water-based systems described previously, charge transport does not presuppose the presence of an aqueous phase.
For example, in the single-phase electrolyte system described in U.S. Pat. No. 5,525,436 (also in contrast with heterogeneous systems), industrially relevant conductivities are reached only at temperatures above 100° C. The conductivity of the polymer membrane depends on the concentration of phosphoric acid contained in it but does not depend to any mentionable extent on the water content. Polymer membranes based on PBI are gastight. For this reason and due to the complexing of the acid by the polymer, the loss of acid from the cell is negligible. The use of corresponding polymer membranes should permit operation of the fuel cell in the working temperature range of >100° C.<200° C., with advantages in the range of catalyst efficiency and low catalyst sensitivity and should greatly simplify water management. For these reasons and because of the high oxidation stability of this polymer, phosphoric acid-doped PBI polymer membranes have been developed as the electrolyte and separator for fuel cells in the working temperature range above 100° C., e.g., for approximately 160° C.
The phosphoric acid-doped PBI polymer membranes are usually manufactured in a multi-step process (see WO 01/18894 A2 or WO 96/13874 A1), with the PBI being dissolved in dimethyl acetamide (DMAc) containing 2% lithium chloride (LiCl) in the first step and the solution being applied in the form of a film by casting or spraying. After drying the film, the LiCl is extracted from the film with water. An amorphous and flexible polymer membrane is obtained as an intermediate product. In the next process step, this polymer membrane is doped with sulfuric acid (H2SO4) or preferably with phosphoric acid (H3PO4). In the process, the polymer membrane absorbs many times its own weight of dopant. The resulting doped polymer membrane is partially crystalline, mechanically unstable because of the high degree of swelling and also difficult to handle. The degree of crystallinity and thus the mechanical instability increase in the presence of water, e.g., in storage or due to the water generated in operation of the fuel cell.
Another way to produce phosphoric acid-doped PBI polymer membranes consists of preparing solutions of suitable monomers and/or prepolymers in polyphosphoric acid, as described in WO 02/081547 A1 or DE 10117686 A1, then to convert these solutions to membrane form through suitable methods and polymerize them by the action of temperatures up to 300° C. and finally to hydrolyze them under the influence of atmospheric humidity.
In principle, the polymer membranes used in the past for production of fuel cells have had a high dopant content. When phosphoric acid was used, the ratio of acid to monomer unit of which the polymer is composed was usually more than 50 wt % H3PO4 (approximately 400 mol %) (U.S. Pat. No. 5,525,436, column 10: 470 mol % corresponding to 55 wt %); WO 01/18894, column 6: approximately 67 wt %). Even higher phosphoric acid contents, up to 85 wt %, can be achieved by film-forming polymerization in polyphosphoric acid (WO 02/081547 A1 or DE 10117686 A1). This leads to highly swollen polymer membranes which undergo deformation and have a mechanical loading capacity that leaves much to be desired, which makes their use in a fuel cell problematic, in particular from the standpoint of cell design and long-term stability.
To improve the mechanical properties, it was proposed in WO 00/44816 A1 that the polymer membrane of PBI should be crosslinked with the help of a crosslinking agent. This makes it possible to limit the decline in mechanical stability associated with doping and to improve the processability of the polymer membrane. However, the mechanical strength is still inadequate for practical use in fuel cells because even with these polymer membranes, a ratio of H3PO4 to the PBI repeating unit of more than 6 (corresponding to approximately 65 wt % H3PO4) was also selected with these polymer membranes (WO 00/44816 A1, Table 2, page 16) to ensure an adequate conductivity of the polymer membrane. All the test results on membrane-electrode assemblies described in the literature are therefore based on the investigation of small cells in which the low mechanical stability of the polymer membrane is not so important and which have an active area of less than 40 cm2 (WO 00/30202 A1, WO 01/18894 A2 and investigations in micro-fuel cells: WO 96/13872 A1).
Membrane-electrode assemblies for polymer electrolyte fuel cells for a working temperature range between 100 and 200° C. with polymer electrolyte polymer membranes based on the PBI-H3PO4 system usually contain two gas diffusion electrodes separated by the polymer membrane.
The gas diffusion electrodes described in the literature for this application have a structure similar to that of electrodes for MEAs with electrolyte membranes based on Nafion, but Nafion is eliminated as an electrode component. The proton conducting connection between the electrode and electrolyte is usually ensured by phosphoric acid. To do so, the electrodes are impregnated with phosphoric acid before assembly of the cell (WO 01/18894 A2). U.S. Pat. No. 5,599,639 is based on the fact that phosphoric acid diffuses out of the polymer membrane into the electrode.
At working temperatures of >>100° C., the loss of phosphoric acid from the cell is minimal. An additional restriction on the phosphoric acid discharge from the cell can be achieved, as proposed in WO 01/18894 A2, PBI is used as the polymer binder for the electrodes, although this is not without problems because of the property of PBI of forming dense films.